fix(tvix/eval): propagate catchable errors at the top of an eval

(Re-)Adds an error variant that wraps a catchable error kind, which is
used for returning the result of an evaluation.

Previously this would return the internal catchable value, which would
lead to panics if users tried to use these. Somehow this was missed; I
think we need error output tests.

Change-Id: Id6e24aa2ce4ea4358a29b2e1cf4a6749986baf8c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10991
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/lib.rs

@@ -301,7 +301,7 @@ where
             nix_path,
             self.io_handle,
             runtime_observer,
-            source,
+            source.clone(),
             globals,
             lambda,
             self.strict,
@@ -310,6 +310,15 @@ where
         match vm_result {
             Ok(mut runtime_result) => {
                 result.warnings.append(&mut runtime_result.warnings);
+                if let Value::Catchable(inner) = runtime_result.value {
+                    result.errors.push(Error::new(
+                        ErrorKind::CatchableError(*inner),
+                        file.span,
+                        source,
+                    ));
+                    return result;
+                }
+
                 result.value = Some(runtime_result.value);
             }
             Err(err) => {